When installing a Point-of-Entry (POE) or whole-house water filter, which is typically installed near where the main supply line enters the property before the pipes branch throughout the house. Plumbing in a filter bypass is strongly recommended.
Selecting a location that has sufficient space to install a bypass system is important; the bypass plumbing allows you to isolate the filter housings as needed making filter changeout much easier. In addition, it provides the option to have a water supply while the filter housing is being serviced. It also provides the option to have water in the event something happens to your filter housing, which could leave you without water.
NOTE: Bypass plumbing is not limited to point of entry filtration, they are also a great solution for Point-of-Use (POU) filters which may be installed under the counter. Again, a bypass system is always recommended, however it can be a bit more challenging due to space limitations.
To avoid the possibility of the bypass creating a dead leg and growing bacteria, it is best practice to install a drain valve so that the bypass can be easily flushed on a regular basis. Or, to establish regular flushing an automated valve on a timer could be installed to ensure that flushing is not forgotten. It is recommended that you thoroughly flush the bypass each time the filter cartridge is changed (6-month intervals maximum is recommended). While most homeowners simply open the bypass valve for a short burst of water to flush the bypass, this does allow a small volume of unfiltered water to pass through to the home plumbing. Best practice is to close the two ball valves on either side of the filter housings while flushing the bypass to ensure unfiltered water does not seep back into the home supply line. The bypass system and flush valve can be installed as per either of the two options shown below.
This risk can be eliminated by avoiding a bypass; the cartridge could be removed if it becomes too clogged and there is no replacement cartridge on hand for immediate changeout.
Additional Resources:
https://support.boshart.com/what-are-the-required-steps-to-properly-select-a-water-filter
https://support.boshart.com/how-to-replace-a-whole-house-water-filter-cartridge